1. Field of the Invention
This invention relates to cooling towers and other direct contact heat and mass transfer devices utilizing fill media. More particularly, the invention relates to an improved film fill for use in cooling towers.
2. Description of the Related Art
Fill is the material over and through which fluids in heat and mass transfer devices flow. Cooling towers utilizing fill are constructed so that films of the liquid to be cooled flow over and coat the fill surfaces while the cooling medium flows through the fill. The liquid to be cooled and the cooling medium flow in different directions and directly contact each other.
Generally, the liquid to be cooled is sprayed over the fill, with the fill being contained within an enclosed space. The cooling medium, e.g., air, is supplied by means of a natural, induced, or forced draft. The draft may be horizontal (i.e., cross current) or vertical (i.e., counter current). The liquid to be cooled flows down, coating the fill, and directly contacts the counter or cross current cooling media. Generally, in counter current cooling, the air enters at the bottom of the tower and travels upward. In general, the greater the surface area of the liquid to be cooled contacting the cooling media, the more efficient the cooling tower will be.
Generally, such cooling towers include a housing through which air is admitted and exhausted by suitable means such as, e.g., exhaust fans. The liquid to be cooled (e.g., water) water) is distributed throughout the housing by a water distribution system (e.g., sprinklers) located above the fill. The water falls by gravity to a basin located at the base of the housing.
Fill can take many forms, including multi-cell blocks, multiple sheet configurations, and multiple plate configurations. Fills can also be made of many different materials including, but not limited to, plastics such as PVC, wood, metals, ceramics and fibrous cement.
Film fills should preferably exhibit the following characteristics:
1. Deploy a large surface area in a relatively small volume, thereby maximizing heat and mass transfer.
2. Allow air and water to pass over and through the fill pack and come into contact with each other with little airflow resistance.
3. Minimize the accumulation of solids, i.e., fouling of fill surfaces.
4. Provide a long service life, preferably in excess of 25 years.
5. Be inert to various water chemistries and insusceptible to UV damage.
6. Be able to withstand freeze-thaw cycling without damage.
7. Be able to operate at water temperatures in excess of 135xc2x0 F. without loss of physical integrity or mechanical strength.
8. Be of rugged construction with the ability to withstand foot traffic on the top surface of the fill without damage or loss of shape.
9. Be non-flammable.
10. Be low in cost.
11. Be lightweight, thereby minimizing structural support requirements.
12. Be comprised of materials that are non-toxic, non-hazardous and suitable for easy and safe disposal at the end of service life.
Film fills currently utilized in cooling towers include multi-plate types comprised of asbestos-cement or fibrous cement plates, multi-sheet types comprised of plastic sheets, and ceramic multi-cell block types. Each of these fills exhibits a number of the aforementioned desirable characteristics while suffering from various drawbacks.
Multi-Plate Asbestos-Cement and Fibrous Cement Fills
Multiple vertically placed asbestos-cement or fibrous cement plates have been extensively employed in cooling towers as film fill. The plates are flat and rectangular, approximately {fraction (3/16)} inch thick and are spaced in the range of xc2xd to 2 inches apart. Multiple layers of the plates are typically deployed in cooling towers, carried either directly on beams, or, alternatively, suspended from the beams.
These fills exhibit limitations with respect to service life, attack by various water chemistries and damage caused by freeze-thaw operation. They are also heavy and high in cost. Use of asbestos-cement fills is extremely problematic because they are hazardous and present disposal problems.
These plate type fills have straight top and bottom edges. If alternate layers are stacked parallel to each other in a cooling tower it becomes necessary to install special transverse spacers between layers to maintain separation and provide adequate structural support. The spacers also serve to minimize the flow restrictions occurring at the interface between layers. The parallel stacked arrangement is disadvantageous since internal mixing of the water and air flowing over and through the fill media can take place in one plane only, thereby diminishing the thermal performance of the fill. Additionally, extra costs are incurred relative to the supply and installation of the spacers.
If alternate layers are stacked at right angles to each other, good mixing of both air and water is promoted. However, air side pressure drop is significantly increased because of the restriction in cross-sectional area that occurs where alternate layers contact each other. Spacers between layers can be utilized to avoid the multiple restrictions at the layer to layer interfaces. This, however, increases the cost of the fill.
Fill plates of this type are relatively thick and are flat on both sides. The considerable thickness of the plates increases the size of the obstruction created at contact points, reduces cross sectional flow area and consequently increases air side pressure drop. The thickness of these fill plates additionally increases the weight of the fill with an attendant increase in structural costs. Air side pressure drop is detrimental to cooling tower efficiency since it necessitates increased power consumption to create adequate air velocity.
In some cases, plates of this type are assembled into multi-plate assemblies, also referred to as fill packs. In those cases, plastic spacer combs are utilized to join multiple plates to form a fill pack prior to installation in cooling towers. Assembly is accomplished by inserting combs through slotted openings in the plates and then rotating the combs by 90xc2x0. The spacer comb design has several shortcomings including the limited service life of the plastic comb material, reduction in strength at elevated temperatures and restrictions in available comb pitch settings.
Plate type fills made of asbestos-cement or fibrous cement absorb significant amounts of water when in service. Absorption of water subjects plates of this type to freeze-thaw damage with an attendant decrease in useful service life. Additionally, the cementitious makeup of these plates and the use of cellulose fibers in fibrous cement precludes the use of these plates in certain water chemistries due to the reactive nature of these materials.
Multi-Sheet Plastic Fills
Multiple vertically placed plastic sheets are frequently employed in cooling towers as film fill. The sheet material most commonly used is polyvinyl chloride (PVC). The sheets are usually corrugated with the flute angle of the corrugations inclined typically about 60xc2x0 from the horizontal. Adjacent sheets are cross-stacked with respect to each other so that the corrugations serve as a spacing means for the sheets. Other spacing configurations and sheet topographies are also employed depending on the application. The sheets are typically attached to each other with a bonding material at the locations where they contact each other thus forming fill packs. The sheets are very thin, typically in the range of 0.008 to 0.015 inches.
Multi-sheet assemblies formed of plastics such as PVC provide only a limited service life, are flammable, have low structural strength, and can present disposal problems. Additionally, plastic fills cannot be operated at high water temperatures, i.e., in excess of 135xc2x0 F. without risk of structural deformation unless high temperature plastics such as CPVC are utilized. Use of high temperature plastics such as CPVC can result in as much as a doubling of fill costs. Fills of this type can be made rugged enough to withstand foot traffic on the top surfaces of the fill only if the fill sheet thickness is significantly increased. Such measures increase the cost of the fill. Due to the flammable nature of plastic fills, fire protection systems must, generally, be installed in cooling towers utilizing plastic fill. The cost of installing fire protection systems further increases the cost of utilizing plastic fill.
Integral spacing features between adjacent layers of fill have been employed in prior art plastic sheet fills, principally to reduce the fouling potential of the fill. The spacing features were created by alternately nesting longer and shorter sheets adjacent to each other. Such a structural configuration is disadvantageous since only every other sheet in the fill pack is load bearing, thus greatly increasing the contact stresses in the extremely thin sheets.
Individual plastic fill sheets are generally assembled into multi-sheet fill packs prior to installation in cooling towers by gluing or heat bonding select portions of the sheets together. These methods of joining plastic fill sheets have proved unreliable in many cases.
Ceramic Multi-Cell Blocks
Ceramic blocks stacked directly on top of each other, each typically 12 inches square and 6 inches tall, are also employed as film fill in cooling towers. The blocks are typically partitioned into multiple open 2 inch square cells with the cell walls being between xc2xc and xe2x85x9c of an inch thick. Water flows downward, coating the cell walls, and the cooling medium (air) flows upward through the cells counter-current to the water. The blocks are extruded in the wet clay stage and are then fired. The blocks are relatively heavy because of the thick cell walls that are demanded by the manufacturing process.
Due to material and process limitations, ceramic multi-cell blocks deploy a relatively small amount of surface area per unit volume, resulting in low heat transfer efficiency. The blocks are also very heavy due to the thick walled cell construction that is employed. These disadvantages severely limit the use of ceramic multi-cell blocks.
Providing a gap between adjacent layers of blocks has been found to be beneficial to reduce air side pressure drop. This has been accomplished by installing separate spacers between adjacent layers of flat sided blocks or by recessing most of the interior area of one side of the blocks. This has the disadvantage of removing significant surface area from a fill pack having an already low surface area. Another disadvantage of ceramic fill blocks is that the cellular configuration permits only vertically channeled flow thereby decreasing the mixing efficiency of the liquid to be cooled and the cooling medium.
What is needed in the art is a cooling tower fill which exhibits the aforementioned desirable attributes while being free of the disadvantages exhibited by the fill currently in use.
The fill plates of the present invention are comprised of ceramic material and are generally rectangular in configuration. The ceramic material of construction can be, e.g., cordierite, zirconia, alumina, mullite, porcelain, semi-porcelain, stoneware and earthenware. The fill plates of the current invention are assembled into multi-plate assemblies, also referred to as fill packs, utilizing novel spacer elements providing substantially unlimited freedom to set spacing, i.e., pitch distances between plates. The plates within a fill pack are arranged substantially parallel to each other with the spacing between plates being completely adjustable to allow optimization of performance. Individual plates employ a ribbed design which makes them relatively lightweight and yet strong. Individual plates also incorporate integral gapping features along the top and bottom edges of each plate which reduce the number of contact points between the layers of stacked fill packs and therefore minimize air side pressure drop through the fill media.
The invention, in one form thereof, comprises a fill pack suitable for use in cooling towers. The fill pack of this form of the current invention is comprised of a plurality of substantially planar plates. Incorporated in the plates is a matrix of integral interconnected ribs. The rib pattern can take many forms including a plurality of squares, rectangles, circles, or hexagons. Ribs also extend over the entire perimeter of the plates thus increasing the ruggedness of the plates. The width of the perimeter and interior ribs is, e.g., 0.25 inches and 0.12 inches respectively. The thickness of the ribs and interconnecting plate ligaments is, e.g., 0.12 and 0.06 inches respectively. The ribbed pattern may be incorporated on one or both sides of the plates. In addition to minimizing weight, the purpose of the ribs is to increase airflow turbulence, to increase the dwell time of the water in the fill zone and to promote uniform water distribution. All of these attributes increase heat transfer efficiency.
In one form of the present invention, each plate of the fill pack includes two opposing contact edges. The contact edges of each plate include a plurality of recessed portions. The recessed portions can be uniformly spaced on each contact edge.
In addition to the ribs, each plate incorporates a number of circular apertures that penetrate the plate on a generally uniform pattern. Each aperture is typically 0.375 inches in diameter and is reinforced by an integral circular rib. The fill pack of this form of the current invention further includes separate fixing devices which securely hold and space a number of plates apart from each other at a pre-selected distance, thus constituting a fill pack. The fixing devices are inserted through the plate apertures and include engagement portions that fix the plates at the desired spacing. In one form of the current invention, the engagement portion between plates comprise a flattened tube section. In an alternate form of the invention, the engagement portions comprise an expanded tube section. In one form of the current invention, the fixing device comprises a corrosion resistant metal tube, formed of, e.g., copper. In other forms of the current invention, the fixing device can be, e.g., formed of a solid plastic, e.g., polyvinyl chloride, rod or tube.
The invention, in another form thereof, comprises a method for forming a fill pack for use in a cooling tower. The method of this form of the current invention includes the steps of: placing a plurality of plates each of which has an aperture in a jig, traversing the apertures of the plates with a tube, and deforming the portions of the tube which occupy the space between adjacent plates.
In one form of the current invention, the step of deforming the portions of the tube between the plates comprises heating and applying compressive force to the portion of the tube to be deformed. In anther form of the current invention, this step can comprise the steps of: providing a swaging tool, and swaging the portions of the tube which are to be deformed. The swaging tool can be, for example, a hydraulic swaging tool.
An alternative method for creating fill packs from ceramic plates would be to replace the apertures with a number of integral protrusions (i.e., dimples) during manufacture. To assemble plates into fill packs, the protrusions on each plate would meet and bond with the next adjacent plate or opposing protrusion on the next adjacent plate. The depth of the protrusions could be varied to achieve the desired spacing between plates. The interface adhesion would be suitable epoxy or a ceramic to ceramic type bonding joined during the processing of the plates (i.e., glass adhesion). An epoxy bond, however, would have the same limitations as plastic materials in cooling tower service.
The invention, in another form thereof, comprises a method of placing a plurality of fill packs in a cooling tower. The method of this form of the current invention comprises the steps of: providing a plurality of fill packs, each of which is formed from a plurality of plates having a pair of periodically recessed edges; and placing alternate layers of fill packs at right angles to each other. This results in significantly fewer points of contact between layers and lower air flow resistance than would occur if the plates had straight edges.
Cooling towers may be of the counter-flow type where the cooling medium, e.g., air travels in a direction opposite to the descent of the water or of the cross flow type where the air travels in a direction transverse to the descent of the water. The improved film fill of this invention is applicable to both types of towers and is, in general, applicable to all types of towers in which water is to be cooled.
In addition to application in cooling towers, the improved fill of this invention is also applicable for use in other applications such as in trickle filters of water treatment plants where it can be employed to expose large amounts of wetted surface to flowing air to oxygenate the water and aid the digestion process.
An advantage of the present invention is the ability to substantially lessen the weight of plate type cooling tower fill while maintaining the desired strength of the fill.
Another advantage of the present invention is the ability to form multiple plate cooling tower fills with a plate-joining element which allows substantially complete adjustability of the spacing (i.e., pitch) between adjacent plates.
Yet another advantage of the present invention is the ability to lessen the points of contact between fill packs stacked one on top of the other, thus lessening air flow resistance and air side pressure drop through the fill.
A further advantage of the present invention is the ability to provide a fill structure which somewhat increases air flow turbulence in the fill pack, thereby increasing heat transfer efficiency.
Another advantage of the present invention is the ability to provide a fill pack that promotes uniformity of water distribution and which increases-the dwell time of the water within the fill pack.
Yet another advantage of the present invention is the ability to create fill packs having a high resistance to buckling.
Yet a further advantage of the present invention is the ability to create a ceramic fill having a high strength-to-weight ratio and low air flow resistance while maintaining the advantages of ceramic fills (i.e., low fouling, long life, inertness, ability to withstand freeze-thaw cycling without damage, ability to operate at high temperatures, and non-flammability).